Although external and internal slat-type blinds (such as venetian blinds) are used frequently to reduce the solar load on windows, there are no detailed calculation models for such shading devices within detailed thermal simulation models (DSMs). A new calculation model (GLSIM-BLIND) has been written to either simulate blind systems accurately at each simulation time-step, or to simulate blinds approximately using simplified daily effective shading factors for each radiation component. In addition, the program calculates annual or monthly effective total solar transmittance values, or effective shading factors, which can be used in thermal calculations or in simple dynamic programs. The model can be used for a wide range of different blind systems and blind control strategies. It is suitable for optimising blind arrangements during the design phase of a building. This article shows the theoretical basis of the new calculation technique and demonstrates its application in thermal building simulations.